<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">一次</button>
    <script>
        function once(fn){
            var flag = false //默认没有被执行
            return function(){
                if(!flag){
                    //修改this指向
                    fn.apply(this,arguments)
                    flag = true
                }
            }
        }
        document.getElementById('btn').onclick = once(
            function(event){
                console.log('hah')
                console.log(this)
                console.log(arguments)

            }
        )
    </script>
</body>
</html>